Surface and subsurface stratigraphy of the Santa Fe Group near White Rock and the Buckman areas of the Espanola Basin, north-central New Mexico
Daniel J. Koning, David Broxton, David Sawyer, David Vaniman, and John Shomaker
Numerous wells in addition to stratigraphic sections of surface exposures provide a robust data set to examine Santa Fe Group stratigraphic relations near the White Rock and Buckman areas of the Española Basin. Here, wells penetrate Santa Fe Group strata, ranging in age from ca. 13.5 to 8.5 Ma, that locally underlie Plio-Pleistocene strata. Surface exposures of Santa Fe group strata range in age from ca. 12 to 8.5 Ma. Santa Fe Group strata are characterized by fluvial deposits of sandy to gravelly channel-fills intercalated with floodplain deposits of clay, silt, very fine- to fine-grained sand, and silty sand. Most of the aquifer under the Buckman well field consists of ancestral Rio Grande fluvial deposits belonging to the Vallito Member of the Chamita Formation, which overlies finer-grained, basin-floor deposits of the Pojoaque Member of the Tesuque Formation. The Vallito Member is a very pale brown to pink to light gray unit dominated by subrounded (minor rounded and subangular), relatively clean, chert- and volcanic-bearing, quartz-dominated sand that is locally frosted. Vallito Member gravels consist of very fine to coarse pebbles. The Vallito Member interfingers westward with light gray volcaniclastic sediment of the Hernandez Member (Chamita Formation) west of Buckman, deposited by an ancestral Rio Chama derived from the northwest that flowed alongside and merged with the ancestral Rio Grande. A high degree of mixing occurs between the Vallito and Hernandez Members within 1-4 km of their interfingering zone, which extends about 7 km to the west of the Rio Grande at Buckman. The gravel fraction of the Hernandez Member in the study area includes very coarse pebbles and cobbles, and is dominated by subrounded to rounded, dark gray to greenish dacites-andesites with less than 15% quartzite. Locally, the Vallito Member overlies, and interfingers eastward with, fluvial deposits of the Cejita Member of the Tesuque Formation. However, it appears that the northeast-derived river associated with the Cejita Member merged with the ancestral Rio Grande north of Buckman. More commonly, the Vallito Member interfingers eastward with granite-bearing alluvial-slope deposits of the Cuarteles Member of the Chamita and Tesuque Formations. As is the case throughout the Española Basin, the Cuarteles Member here progressively prograded westward in the middle to late Miocene. A probable angular unconformity and a general down-section increase of dips indicate that the Santa Fe Group in the study area was deposited during active west-tilting of the Española Basin halfgraben. We interpret a westward increase of stratal tilts as related to subsidence-related flexure on the eastern side of an intrabasin half-graben, marked by a pronounced low Bouguer gravity anomaly. The lack of significant playa or lacustrine deposits in these strata indicates that closed basin conditions did not exist during 8.5 to 13.5 Ma.
- Koning, Daniel J.; Broxton, David; Sawyer, David; Vaniman, David; Shomaker, John, 2007, Surface and subsurface stratigraphy of the Santa Fe Group near White Rock and the Buckman areas of the Espanola Basin, north-central New Mexico, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 209-224.